Conventionally, there is a known gas engine drive system including a gas engine and a turbocharger (see Patent Literature 1, for example). The turbocharger includes a compressor and a turbine that are connected to the gas engine. The gas engine is provided with a fuel injection valve, and a fuel gas is led to the fuel injection valve through a fuel supply line. The fuel supply line is provided with a pressure regulating valve. Generally speaking, the pressure regulating valve is controlled such that the pressure difference between the fuel supply pressure and intake air pressure is constant.